1. Field of the Invention
This invention pertains to aerodynamic "wind" vanes used for sensing the direction of air flow along the exterior surface of the fuselage of an aircraft as part of a stall warning system. The stall warning system provides a warning to the pilot by means of a "stick shaker" or a warning horn when the flight attitude of the aircraft is approaching an aerodynamic stall. In response to the warning, the pilot can then alter the attitude of the aircraft so as to avoid the impending aerodynamic stall .
2. Description of the Prior Art
FIGS. 1--3 depict sensors that have typically been used in the prior art to sense the angle of attack of an aircraft. For the purpose of illustration, the physical size of the sensors has been greatly exaggerated in these figures relative to the size of the aircraft. Referring to FIGS. 1 and 2, port sensor 1 and starboard sensor 2 are rotatably attached to the exterior surface of the fuselage 3 of aircraft 4. Sensors 1 and 2 are located on opposite sides of the fuselage and their axes of rotation may or may not coincide with each other depending upon whether the exterior surfaces of the fuselage at the points of attachment are parallel to each other. Sensor 1 consists of an arm 5 that is rotatably attached at its end 6 to the fuselage 3 so that it has an axis of rotation 7. A vane 8 is attached to end 9 of the arm.
As depicted in FIG. 2, in the prior art, vane 8 typically has a wedge-shaped or triangular cross section and plane 17 passing through the center line 10 of the vane 8 also passes through and contains the axis of rotation 7.
During flight, sensor 1 responds in the manner of a wind vane to the flow of air along the exterior surface of the aircraft and the angular position of the vane is indicative of .alpha., the angle of attack of the aircraft as depicted in FIG. 2.
In the prior art, for the purpose of providing reliability, two sensors are used, one located on each side of the aircraft. The angular position of each sensor is then compared with the other as a means of confirming the accuracy of the indications. This comparison provides a reliable means of confirmation when the yaw angle of flight is substantially zero, i.e. when the aircraft is moving straight forward through the air. However, for a non-zero yaw angle of flight as depicted in FIG. 5, the air stream is asymmetrical relative to the aircraft and causes the port sensor 1 to exhibit an angular position differing from that of the starboard sensor 2.
A third sensor located on the top or the bottom of the fuselage has been used in the prior art to sense the angle of yaw. The yaw angle is then used to predict the difference in angle between the port and starboard sensors that would be expected in normal operation during yawed flight and thereby provides a means for confirming the proper operation of the sensors. The addition of a yaw vane, however, complicates the system and reduces the overall reliability of the stall warning system.